In the prior art it is well known to solder, fuse or braze articles by condensing hot saturated vapor of a heat transfer liquid thereon. Typically such operations are performed in a chamber having a reservoir of the heat transfer liquid therein. Immersion heating coils in the reservoir are activated to boil the liquid while cooling coils mounted in the upper portion of the chamber are used to condense vapor from the boiling liquid to form a body of hot saturated vapor and substantially preclude the escape of the vapor from the chamber. The articles are then immersed in the body of vapor which condenses thereon and gives up its latent heat of vaporization thereto to heat the articles to the temperature required for soldering, brazing or fusing. Such a technique is described in U.S. Pat. No. Re. 30,399 which issued on Sept. 9, 1980 and is assigned to Western Electric Company and Bell Telephone Laboratories, Inc. and which is incorporated by reference herein.
This technique has proven to be eminently successful, however, solder flux placed on the article to be soldered is washed therefrom by the liquid condensate and falls into the reservoir of the boiling heat transfer liquid. The flux and other contaminants coming in contact with the immersion heaters can cause "hot spots" on the heaters which can lead to thermal degradation of the heat transfer fluid.
To overcome these problems the heat transfer liquid-flux mixture dripping from the article being soldered is intercepted by a pan and redirected outside the facility to a filtration system described in copending U.S. patent application No. 278,151 titled "Rosin Flux Filtration System" by E. J. March and G. M. Wenger, filed in the U.S. Patent and Trademark Office on even date herewith and which is incorporated by reference herein. In that system the liquid-flux mixture passes through flux filters and heat exchangers to cool the mixture to precipitate out and remove the flux therefrom.
However, once the flux filtration has been accomplished the temperature of the heat transfer liquid is about 70.degree. F. The liquid cannot be directly fed back into the liquid reservoir of the condensation soldering facility which is boiling at 419.degree. F. for such mixing would result in turbulence and rapid lowering of the temperature of the mixture. Such turbulence and lowering of the temperature of the reservoir liquid can alter the level of the vapor-air interface and possibly cause the body of hot saturated vapor to collapse. Accordingly, the cooled liquid must be heated to an elevated temperature of about 400.degree. F. before being reintroduced into the reservoir of the condensation soldering facility. Furthermore, the liquid is very expensive and it is desirable that any evaporation thereof during heating be minimized.
Accordingly, there is a need for an energy efficient heating means to elevate the temperature of a heat transfer liquid while precluding loss of vapor thereof to the atmosphere.